Do not use jaxen XPath

Do not use jaxen XPath

We are really interested only in Expr and its evaluation, hence we
can skip the BaseXPath class, which is hiding things from us.

This also lowers the memory overhead of compiled expression very
slightly and improves performance, as the result is not needlessly
obfuscated via a singleton list.

Furthermore we no longer go through XPathReaderFactory, which incurs
reflection overhead, but rather instantiate
org.jaxen.saxpath.base.XPathReader directly.

Change-Id: I7fd315c17f4013f927e724226408ba12f8445606
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>